01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Slice the tops off each bread loaf and hollow out the centers, leaving about 1/2-inch thick walls and base to form bowls. Reserve the removed bread for another use.
03 - Mix softened butter with minced garlic, parsley, salt, and Parmesan (if using). Spread this mixture evenly inside each bread bowl and on the cut edges.
04 - Place bowls on the prepared baking sheet. Bake for 10-12 minutes until golden and crisp. Set aside.
05 - Cook spaghetti in salted boiling water according to package instructions until al dente. Drain and set aside.
06 - In a large skillet, heat olive oil over medium heat. Add onion and sauté 2-3 minutes until translucent. Add garlic and cook 1 minute more.
07 - Pour in marinara sauce, oregano, pepper, and salt. Simmer for 5-7 minutes.
08 - Add cooked spaghetti to the sauce. Toss well to coat.
09 - Fill each garlic bread bowl with spaghetti mixture. Top with mozzarella (if using). Return bowls to oven and bake an additional 5-7 minutes, until cheese melts and filling is heated through.
10 - Garnish with fresh basil before serving.